NEUROMAGNETIC MAPPING OF MULTIPLE VISUAL AREAS IN HUMANS

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Noninvasive magnetic recordings of human brain activity are combined with anatomical MRI and mathematical models for electromagnetic source estimation to investigate the locations and arrangement of multiple visual areas in human cortex and to probe their functional characteristics. Results obtained during the preceding project period suggest that selective manipulation of visual stimulus properties (e.g., color and luminance) can preferentially activate sources in humans consistent with the "ventral" versus "dorsal" processing streams identified in non- human primates (Ungerleider and Mishkin, 1982). These results have been interpreted within a framework of the specific receptive field properties associated with these two streams. Regions of activation evoked by isoluminant chromatic stimuli were localized to lateral lingual and fusiform gyri; in nonhuman primate studies, the fusiform gyrus has been shown to contain extensive representations of the central visual field and to be preferentially sensitive to color. The present application extends previous work by focusing on activity associated with the dorsal stream of processing, which subserves spatial vision. Peripheral field locations will be assessed by presenting small circular sinusoid stimuli ("bulls-eye" patterns) to 3 eccentricities in the upper/lower and left/right visual fields. Automated source localization procedures will be applied to these neuromagnetic measures to identify and localize the structures responsive to these stimuli. Functional characterization of structures in the dorsal stream will be assessed by: 1) manipulating the temporal frequencies of alternation between white/gray bands comprising the circular sinusoids and 2) by manipulating the speed at which red/green and gray/white bands appear to move in an outward direction (expanding motion). In some conditions, the perception of motion will be "nulled" by adjusting the intensity of the red/green bands to equalize perceptual luminance. The long term goal of this project is the delineation of specific information processing pathways in the human brain. The methods and knowledge achieved through functional characterization of the different visual areas will ultimately aid in better understanding of higher cognitive functions.